1. Technical Field of the Invention
The present invention relates to a liquid crystal display device intentionally and preferably designed to reduce power consumption during operation for driving liquid crystal display elements.
2. Description of the Related Art
Recently, an active matrix liquid crystal display device used as a portable terminal monitor has been increasingly required to reduce power consumption during its operation. To date, reduction in power consumption of liquid crystal display device has been made possible such as by reducing power consumption of driver IC and/or improving the efficiency of operation of power supply IC. However, the above-described improvement efforts are now becoming inefficient and therefore, power consumption during operation for driving a liquid crystal panel needs to be reduced.
For example, Japanese Patent Laid-Open No. 10(1998)-293559 discloses a liquid crystal display device configured to reduce power consumption during operation for driving a liquid crystal panel. The conventional liquid crystal display device disclosed in this publication operates such that immediately before the polarity of a voltage on a common electrode is inverted, electric charge accumulated in a liquid crystal display element is collected as a collection voltage having the same polarity as the voltage on the common electrode and supplied to the liquid crystal display element at the time the polarity of the voltage on the common electrode becomes the same as that of the collection voltage. The liquid crystal display element acts as a capacitor and discharges current generated when the polarity of a terminal voltage across the liquid crystal display element is inverted. The energy generated is stored in a coil, and the current generated by discharge from the coil is rectified. Then electric charge accumulated in the capacitor upon activation of the liquid crystal display element is collected as a voltage having the same polarity as the voltage on the common electrode by a capacitor of a charge collection circuit. The electric charge collected by the capacitor is again supplied (re-supplied) to the liquid crystal display element at the time the common electrode is driven to a voltage with the same polarity as the collection voltage.
However, the conventional technique disclosed in the publication has the following drawbacks. That is, the liquid crystal display device according to the conventional technique basically operates such that energy generated when the common voltage VCOM is changed is stored in the coil via a capacitor (capacitor between a pixel electrode and a common electrode) of the liquid crystal display element as and current generated by discharge from the coil is rectified and accumulated in a collection capacitor, resulting in reuse of the electric charge. However, since capacitance associated with the common electrode (i.e., capacitance between a common electrode and a gate electrode, between a common electrode and a drain electrode, and between a common electrode and the ground, and further including stray capacitances) is large, change in voltage between both terminals of the coil becomes smaller, unfavorably resulting in lowering of collection ratio of electric charge within the liquid crystal display device.
Furthermore, since a voltage to be applied to the pixel electrode is applied thereto via the drain electrode and then a TFT, a time constant of the voltage becomes large and accordingly, change per time in voltage between both terminals of the coil becomes small, unfavorably resulting in lowering of collection ratio of energy within the liquid crystal display device.